Memory expansion is one of the many fields where high density circuit module solutions provide space-saving advantages. For example, the well-known DIMM (Dual In-line Memory Module) has been used for years, in various forms, to provide memory expansion. A typical DIMM includes a conventional PCB (printed circuit board) with memory devices and supporting digital logic devices mounted on both sides. The DIMM is typically mounted in the host computer system by inserting a contact-bearing edge of the DIMM into a card edge connector. Typically, systems that employ DIMMs provide limited profile space for such devices and conventional DIMM-based solutions have typically provided only a moderate amount of memory expansion.
As bus speeds have increased, fewer devices per channel can be reliably addressed with a DIMM-based solution. For example, 288 ICs or devices per channel may be addressed using the SDRAM-100 bus protocol with an unbuffered DIMM. Using the DDR-200 bus protocol, approximately 144 devices may be addressed per channel. With the DDR2-400 bus protocol, only 72 devices per channel may be addressed. This constraint has led to the development of the fully-buffered DIMM (FB-DIMM) with buffered C/A and data in which 288 devices per channel may be addressed. That buffering function is provided by what is typically identified as the Advanced Memory Buffer or AMB. With the FB-DIMM, not only has capacity increased, pin count has declined to approximately 69 signal pins from the approximately 240 pins previously required.
There are several known methods to improve the limited capacity of a DIMM or other circuit board. In one strategy, for example, small circuit boards (daughter cards) are connected to the DIMM to provide extra mounting space. The additional connection may, however, cause flawed signal integrity for the data signals passing from the DIMM to the daughter card while the additional thickness of the daughter card(s) increases the profile of the module.
Multiple die packages (MDP) can also be used to increase DIMM capacity. This scheme increases the capacity of the memory devices on the DIMM by including multiple semiconductor die in a single device package. The additional heat generated by the multiple die typically requires, however, additional cooling capabilities to operate at maximum operating speed. Further, the MDP scheme may exhibit increased costs because of increased yield loss from packaging together multiple die that are not fully pre-tested.
Stacked packages are yet another way to increase module capacity. Capacity is increased by stacking packaged integrated circuits to create a high-density circuit module for mounting on the larger circuit board. In some techniques, flexible conductors are used to selectively interconnect packaged integrated circuits. Staktek Group L.P. has developed numerous systems for aggregating CSP (chipscale packaged) devices in space saving topologies. The increased component height of some stacking techniques may, however, alter system requirements such as, for example, required cooling airflow or the minimum spacing around a circuit board on its host system.
The present assignee, Staktek Group L.P., has also developed a number of new technologies for circuit modules that can supplant traditional DIMMs. Examples of these new circuit module constructions are shown in the several patent applications incorporated by reference herein. These new technologies populate flex circuitry with integrated circuits and, in preferred embodiments, dispose the IC-populated flex circuitry about a rigid substrate. Edge connector contacts are disposed along the flex circuitry to provide a connective facility for the module which, through these new constructions, provides increased capacity as well as, typically, thermal advantages. With the added IC capacity however, there typically comes a high internal density requirement for the flex circuit that may be implemented with multiple layer flex circuitry. In preferred modes, however, the flex circuitry transits through at least one bend about the end of the rigid substrate and typically passes through at least another arcuate path around a flex support that is typically part of the rigid substrate as shown in a variety of the patent applications incorporated by reference herein. These passages through one or more bends can, in some cases, be problematic where flex circuitry is employed to implement the variety of connections implicated by a complex high capacity circuit module devised to supplant a contemporary DIMM. Consequently, what is needed is a construction and technique to ameliorate difficulties in low profile, high capacity flex circuit based circuit modules.